This invention relates to an articulated robot, and particularly to an articulated robot having plural driving shafts and driving motors which drive the driving shafts.
Various kinds of articulated robots are used for various functions such as arc welding, material handling, coating, and the like. In the present application the invention will be described with reference to an articulated arc welding robot; however, the present invention is not limited only to articulated arc welding robots.
In FIG. 1 an articulated arc welding robot is shown to comprise a swivel stand 12 which is rotatably disposed on a fixed stand 10, which supports the robot body, and connected to the swivel stand 12 is an upper arm 14.
The upper arm 14 is connected at one end to a link 18. The other end of the upper arm 14 is connected to a forearm 22, and a welding torch 24 is supported at the tip of the forearm 22. A link 20 is connected between the link 18 and the forearm 22.
In addition, the above-mentioned swivel stand 12 is driven by a driving motor (not shown), mounted in the fixed stand 10, via a reduction gear, and the welding torch 24 is driven by a driving motor 26.
In a conventional articulated arc welding robot, the driving motor 26 is mounted on the side of the swivel stand 12, and the motor 26 projects therefrom, as shown in FIG. 2. As a result, the driving motor 26 enters into the mounting region of the jig, and thus a disadvantage of the conventional robot is that the movement region of the robot becomes narrow, as shown by the chain line in FIGS. 1 and 2.
In other words, the projecting part of the driving motor 26 is arranged such that it contacts the jig, to which an article to be welded is attached, when the swivel stand 12 is rotated. Therefore it becomes necessary to determine the movement region of the robot in which the motor 26 will not contact the jig. In order to prevent contact between the jig and motor 26, the movement region must be narrowed.
Other conventional arc welding robots are constructed so that the driving power of the driving motor 26, disposed at the side of the swivel stand 12, is transmitted to the welding torch 24 via a chain, a timing belt or the like. However, repeated use of these robots with quick acceleration and deceleration produces a backlash in the chain, timing belt or the like. As a result, the position-determining precision of the welding torch 24 is lowered, and to compensate for this, the upper arm 14 and the forearm 22 must be large-sized. Therefore, in the conventional robot, high-speed movement of the robot is limited, and the interference region of the robot with the jig is larger when the robot is being swivelled.
Another conventional articulated arc welding robot is shown in FIG. 3. This robot includes four wrists, which have been designed for several purposes, namely, optimizing welding pose, using a TIG arc welding robot, using a sensor for detecting a welding line, or the like.
In FIG. 3, a swivel stand 112 is rotatably disposed on a fixed stand 110, which supports the robot body, and an upper arm attaching stand 114 is secured on the upper surface of the swivel stand 112. The swivel stand 112 can be rotated in the directions shown by arrows A and B. An upper arm 118, at one end, is supported in bearings 116 located on the upper surface of the upper arm attaching stand 114, and the upper arm 118 can be tilted in the directions shown by the arrows C and D. A forearm 124 is rotatably supported in bearings 120, located at the other end of the upper arm 118, by a shaft 122, and a welding torch 126 is supported at the tip of the forearm 124.
The forearm 124 comprises sequentially connected plural wrists 128, 130, 132 and 134. In other words, at the bearings 120 of the upper arm 118 is supported one end of the first wrist 128 which is rotatable in the directions shown by arrows E and F about the shaft 122; at the other end of the first wrist 128 is disposed the second wrist 130 which is rotatable in the directions shown by arrow G; at the lower end of the second wrist 130 is disposed the wrist 132 which is rotatable in the directions shown by arrow H; at the third wrist 132 is disposed the fourth wrist 134 which is rotatable in directions shown by arrow I; and the welding torch 126 is supported at the tip of the fourth wrist 134.
The following driving mechanisms are disposed at respective parts of the robot for the purpose of driving the swivel stand 112, the upper arm 118 and wrists 128, 130, 132 and 134 of the forearm 124.
Accordingly, a driving motor (not shown) is mounted in the fixed stand 110 for rotating the swivel stand 112, and the driving force of the motor is transmitted to the swivel stand 112 through chains, timing belts or the like. The driving motors 136 and 138 are disposed at the bearing 116 and are used for rotating the upper arm 118 and the first wrist 128, respectively. The driving forces of motors 136 and 138 are transmitted to the upper arm 118 and first wrist 128 through chains, timing belts or the like.
Furthermore, a driving motor (not shown) is mounted in the first wrist 128 and is used for rotating the second wrist 130. The driving force of the driving motor is transmitted to the second wrist through chains, timing belts or the like.
Driving motors (not shown) are also mounted in the second wrist 130 and the third wrist 132 and are used for rotating the third wrist 132 and the fourth wrist 134, respectively. The driving forces of the driving motors are transmitted to the third wrist 132 and the fourth wrist 134 via chains, timing belts or the like.
However, in the conventional articulated arc welding robot shown in FIG. 3 the driving motors 136 and 138 are disposed so that they project from both outer sides of the bearing 116, in the same manner as the conventional robots shown in FIGS. 1 and 2, and therefore the driving motors 136 and 138 enter into the movement range of the robot and the mounting region of the jig. As a result, the movement region of the robot is narrowed.
Because the driving forces of the driving motor 138 and the driving motor mounted in the first wrist 128 are transmitted to the first wrist 128 and the second wrist 130 through chains, timing belts or the like, the repeated use of the robot with quick acceleration and deceleration produces a backlash in the chain, timing belt or the like. As a result, the position-determining precision of the welding torch 126 is lowered, and to compensate for this the upper arm 118 and the first wrist 128 must be large-sized. Therefore, the high-speed movement of this robot is also limited and the interference region of the robot with the jig is larger when the robot is being swivelled.
Furthermore, the second wrist 130 of the forearm 124 is disposed to one side of the forearm 124 and is therefore shifted away from the central axis of the forearm, which results in decreased operational efficiency. The decrease in the operational efficiency is caused by the wrist shifts away from the central axis of the robot giving rise to an optical illusion, which interferes with proper positioning of the robot, during robot instructions for setting various target angles via rotation of the wrists.
Additionally, in the conventional arc welding robot, there is a further disadvantage in that the second wrist 130 of the forearm 124 is not used for determining the position of the welding torch 126 at an optional position; but rather the second wrist 130 is disposed so that the tip of the welding torch 126 always takes a certain direction when the upper arm 118 is tilted in the directions of arrows C and D, and therefore only three wrists 128, 132 and 134 substantially work effectively.